A. Technical Field
The present invention relates to energetic systems such as primers, igniters, and detonators, and more particularly to an energetic composite and a method of fabricating the energetic composite comprising reactive particles having a multilayer construction formed by the deposition of reactive layers on a rod-shaped substrate so as to have a rod-contoured geometry, e.g. cylindrically-curved, that is controlled to tailor packing fractions as well as reactivities and reaction energies of the energetic composite.
B. Description of the Related Art
There is a need in energetic applications, such as for example pyrotechnics, heaters, delays, munitions, explosives and propulsion, for reactive substances, compositions, arrangements that can react exothermically in an effective manner. In addition, increases in long-term stability, improvements in the rate and energy of reactions, and the ability to control and tune the rates and energies of reactions are most desired.
In order to increase the reactivity of particulate systems, researchers in the past have developed powders or particles with nanometer scale diameters or dimensions. While more reactive, these powered particles have been known to suffer from surface contamination, agglomeration in larger particles, non-uniform distributions of reactants and densities in multi-powder compacts, variability in particle size, and chemical instability.
A different class of energetic materials, known as reactive multilayer foils and energetic nanolaminates comprising alternating layers of materials with large negative heats of mixing, has largely overcome many of the shortcomings of reactive powders and particles by enabling tuning and control of specific reactant chemistries that enable desired levels of stored energy and specific reactant spacing within the particles that enables a desired ignition threshold. In particular, the various design choices available for layer materials, layer dimensions, overall dimensions, bi-layer periodicity, etc. enable such reactive multilayer foils and energetic nanolaminates to be particularly tuned and controlled. FIG. 6 illustrates a cross-section of a generic energetic nanolaminate sheet construction, indicated at reference character 60, which is preferably a multilayer flash metal foil material that is periodic in one dimension in composition, or in composition and structure. They are fabricated by alternating deposition of two or more metallic materials. Individual layers can be varied in thickness from one atomic layer (˜2 Å) to thousands of atoms thick (>10,000 Å). The total thickness of the multilayer foil is shown as 63 in FIG. 6. And the period of the multilayer foil is the distance (i.e. thickness) of the repeating sub unit structure comprising two adjacent metallic layers, hereinafter referred to as the “bi-layer” (such as 61 in FIG. 6) that makes up the foil. It is notable that also included in each bi-layer is a pre-reaction zone (such as 62 in FIG. 6) which is the interface region between the adjacent layers of the multilayer and is made up of a thin layer of intermetallic product formed during deposition. When a localized pulse of energy such as a small spark or flame or mechanical impact is applied to one end of a multilayer foil, the layers of the foil intermix and release heat (exothermic reaction), thereby creating a self-propagating reaction that travels along the foil at velocities that can exceed 10 m/s with maximum temperatures above 1200° C. The environmentally friendly reactive foils can be used to ignite propellants or explosives in place of hazardous azides and can serve as energy sources for local heaters and blast enhancers.
In many energetic applications, however, a particle geometry is still desired instead of a foil or sheet geometry, such as due to packing considerations for example. In particular, it is often desirable to use reactive particles with specific geometries that enable low packing fractions. Previous methods, such as for example crushing or cutting reactive foils have been employed to form reactive particles. However, the resulting particles are typically not uniform in their geometry, which prevents their packing fraction and hence energy per volume from being easily controlled. And still other methods to form reactive particles have involved forming core/shell particles. However, the resulting particles typically have only two to three shells or layers and thus are very small and very hard to manipulate if the reactant spacing (shell thickness) is only tens of nanometers thick. For ease of handling it is desirable to use reactive particles with geometries that range in thickness, width, length or diameter from a few microns to several hundred microns, and reactive particles with geometries that enable packing or volume fractions ranging from 5% to almost 70%.
It would therefore be advantageous to provide reactive particles (and a method of fabrication) with controlled external geometries that facilitate handling and improve packing, in addition to having reactant layers (e.g. nanoscale layers) within the particles to control stability, reactivity and energy density similar to the tunable properties offered by reactive foils.